Generators

Implements support for ES6 generators with emulation for ES5 via the regenerator library.

 - Added keyword `yield` (when annotated with `delegate` generates `yield*`)
 - Two new compiler options:
   - `--runtime`: Embeds any runtime functions needed by the generated code
   - `--es5-generators`: Emulates generators with a runtime
 - Functional tests now support a *modeline* (`; mjs: configOption, configOption = value`) to set specific options
 - Fixes `findParent`

Now we just need some nice looping and async/await macros to make them really useful :)